Asphere manufacturing. Finding the right grinding+polishing conditions and speeding it up is difficult I think, but often the machine manufacturer has service centers that give advice how to do it for a given job, I also got the impression some of these companies are really quick to respond and can even bring you the tooling as the interviewee says. But you need capital. Canon has been grinding aspherics for SLR's since the early 1970's. If you farm out this kind of job to a company in say the most expensive city (Jena in Germany where Zeiss is), a few years ago a 110 mm diameter surface on N-BK7 I remember cost about 600 Euro to CNC grind, not including the initial setup and programming costs. It deeply depends on the radius of curvature and precision and so on I suppose..... There are devices that can do 16 inches too (see video) I don't have experience with them, max I ever saw with my own eyes was I think 12 inches (I'm not sure now), and at that time the running cost told to me was as I remember 100 USD per hour. As the video shows, the measurement device is integrated to find the zero position of the glass and the rough measurement after grinding and everything...... for some of the devices a laser interferometer is included too, though I never saw an actual one. I saw that it takes about 3 months for technical staff with practically no prior experience with CNC to get up to speed......

Still if you want the best possible quality I would imagine it takes more than 3 months of training with the help of the service center. I seem to remember being told the typical height precision you get out of rapid grinding + some fine polishing afterwards over a >100 mm optic was say 1 um, so getting 1/3 or 1/4 lambda consistently might be...... I dunno, hard? without human intervention and adjustments. And the thermal issues with the glass....

It is nice carrying smaller scopes, but I want the biggest scope I can easily carry. I think I can go bigger than 6". 6" is just dim. Maybe a 2" eyepiece would change that. But resolving M13 would still be averted vision.

I've looked in many 8" SCT and newt scopes. They simply don't pack much punch. My old 4.5" scope especially does not do much on planets. 10" is very noticeably better. The best view of M31 I've seen was in a 12" f4. I don't want to carry the 12", though.

Small scopes offer more portability. Large scopes offer more performance. Light-weight large scopes offer more portability at the expense of the shakes. You can have anything you want, but you can’t everything you want in one scope.

Many people own more than one scope for this reason. Other people prefer to own one scope, know themselves well enough, and have enough hands-on experience with scopes to select one scope that will satisfy them well enough for a time, but never perfectly or for all time. So far, no one has built or bought the perfect scope that does everything well.

Asphere manufacturing. Finding the right grinding+polishing conditions and speeding it up is difficult I think, but often the machine manufacturer has service centers that give advice how to do it for a given job, I also got the impression some of these companies are really quick to respond and can even bring you the tooling as the interviewee says. But you need capital. Canon has been grinding aspherics for SLR's since the early 1970's. If you farm out this kind of job to a company in say the most expensive city (Jena in Germany where Zeiss is), a few years ago a 110 mm diameter surface on N-BK7 I remember cost about 600 Euro to CNC grind, not including the initial setup and programming costs. It deeply depends on the radius of curvature and precision and so on I suppose..... There are devices that can do 16 inches too (see video) I don't have experience with them, max I ever saw with my own eyes was I think 12 inches (I'm not sure now), and at that time the running cost told to me was as I remember 100 USD per hour. As the video shows, the measurement device is integrated to find the zero position of the glass and the rough measurement after grinding and everything...... for some of the devices a laser interferometer is included too, though I never saw an actual one. I saw that it takes about 3 months for technical staff with practically no prior experience with CNC to get up to speed......

Still if you want the best possible quality I would imagine it takes more than 3 months of training with the help of the service center. I seem to remember being told the typical height precision you get out of rapid grinding + some fine polishing afterwards over a >100 mm optic was say 1 um, so getting 1/3 or 1/4 lambda consistently might be...... I dunno, hard? without human intervention and adjustments. And the thermal issues with the glass....

Yeah those videos make it look so easy...

but it ain't like that.

cool logo video how they straighten out the fringes...and that is how it does happen over many tests..and figuring stages.

read this link...this is how a machine parabolizes

you want to control it by a computer...does pretty much the same thing...

lots of position changing and not staying on any zone for much time...ie technique.

Thanks I would love to learn these techniques someday.....

The older polishing and grinding machines I have seen at the beginning of my career often mimicked the motions of human polishers and grinders, so it had some large flat tool like a traditional pitch lap that was suspended over the optic, moving back and forth over its whole surface, with many interruptions and measurement steps in between based on a trial-and-error strategy, so the human operator's ability seemed to be very important as you say (I use mostly flat or spherical optics, only a few aspherical or elliptical). You could see by eye how the grinding or polishing was going on, I liked to visit those places.

But these newer machines I saw were similar to the tradition of metal machining (instead of controlling the shape of slopes, it is thinking in terms of removing Z microns from the surface at position X-Y-R-theta-phi)...... so had a very large circular grinding tool that was rotating rapidly, and suspended vertically with respect to the optic, contacting a small area at a time. It moved in a completely different way than shown in your web page, it did not make large strokes across the whole optic, instead it concentrated on smaller areas at a time. Only a small number of tool passes were needed as I remember...... like 3 or 4 (coarse fast - tool change - fine slow), even I could do 2-3 micron and a shiny surface! I was so happy.

There was not a lot of possibilities to adjust the machine during machining - I cannot see the workpiece because there is a protection door and the room inside is flooded with a shower of coolant..... and even if I press my eye against the window, sometimes the tool is far away from the window so I cannot see anything. If I get worried and open the door the safety trips the machine mid-program, and even can damage the workpiece and tool if I do it at the wrong timing, so everything was done automatically ("door open" was the last command of the program). Door could be bypassed and kept open, but I was always told then the temperature gradient could make the machining precision worse, it is a safety hazard, and the coolant could mess up the floor. During machining, I was looking mostly at the computer display, which sequence was now active. I could interrupt or modify the program in real time, but I avoided that because often I would make a mistake in my haste. Maybe I am wrong; you are right, I do not know how they do the last polishing passes to get sub micron figures. But you see the advantage of this method is that many kinds of aspheric shapes could be created and tested, not only paraboloids or hyperboloids. Even arches or half-cylindrical, half-spherical shapes could be created with similar high precision.

I could simulate how the optic would be machined using CAM software. Measurement was like, "wait, select measurement tool, goto coordinate position, measure". I know it is more complicated than that....... but I could easily measure 1000 height points if needed and spit out a data file, and control the next pass automatically to compensate..... though I did not do that. I was always nervous that I might collide the stylus or laser device to something (the window is so small!)..... and well it was implied I should not play with the machine anymore. The machine I used did not have a laser interferometer to measure the optic, so I don't know how that is put into the loop. Once the program and prescription were fully set up, however, many identical parts could be ground. Of course my programming was terrible so many failures and damaged workpieces.....

To grind a very clear surface, I remember the selection of the diamond tool and its angle and adjustment were extremely important, if wrong the surface would lose smoothness, and we let the supplier adjust this and changed it frequently. This tooling was horribly expensive as I recall.

I was told in a factory, maybe 1 person could be looking at 3 or 4 machines as they simultaneously ground optics. This part is speculation, but the single company we are discussing about now: mass-produces Schmidt correction plates and Cassegrain mirrors (5, 8, 9.25, 11 inches) and Newtonian mirrors (3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20 inches) and Ritchey-Chretien mirrors, as well as all diameters of achros and apos (80, 100, 120, 150 doublet and triplet) using ED glasses, fabrication of lenses for eyepieces, also the AR and HR coatings of all optics, design, fabrication, and assembly of the telescopes and computerized GOTO precision mounts and eyepieces; all this without any production delays at extremely low cost, most of the important tasks within 1 factory according to reports. Is it natural to assume armies of trained opticians are doing this? Specialists of mirror fabrication are often not the specialists of lens or prism fabrication, then there would be too much redundancy and one would need to fire or hire people based on the production needs. It would seem to make more sense that there are a few highly trained specialists who initially design the CNC routine and set up the machine and prescription, then the operators do not need to know the tiny specifics. When a production run of a certain optic is needed, the program and maybe part of the tooling is exchanged........ I guess? Otherwise such a huge number and variety of products could normally not be maintained, at such an incredibly low price point and relatively high quality.

Once the program and prescription were fully set up, however, it was a very mechanical process, many identical parts could be ground fast. Of course my programming was terrible so many failures. I was told in a factory, maybe 1 person could be looking at 3 or 4 machines as they simultaneously ground optics. This part is speculation, but this one company mass-produces Schmidt correction plates and Cassegrain mirrors (5, 8, 9.25, 11 inches) and Newtonian mirrors (3, 4, 5, 6, 8, 10, 12, 14, 16, 20 inches) and Ritchey-Chretien mirrors, as well as all diameters of acrhos and apos, without any production delays at extremely low cost, mostly within 1 factory according to reports. Is it natural to assume armies of trained opticians are doing this? I think it is more automatic.....

go follow stellarvue on fb...they show their operation. and Vic talks about that too. but they're doing apo and achros...

but my point is once you polish and test an optic and see the errors...its not just throw it back on the machine. it takes technique. wether by hand or machine.

go follow stellarvue on fb...they show their operation. and Vic talks about that too. but they're doing apo and achros...

but my point is once you polish and test an optic and see the errors...its not just throw it back on the machine. it takes technique. wether by hand or machine.

Thanks! What I saw on their facebook page is a Haas vertical milling machine center, normally used for metals though it can be used for all kinds of materials including glass too... but it cannot so easily produce an optical finish as far as I know by simple grinding (maybe I'm wrong?).... As far as I understood the facebook page, the company uses it for rough cutting and machining of their objective lenses (?) then the lens is removed and a (non-CNC) traditional grinding machine is used, followed by a (non-CNC) polishing machine. This ensures a high quality product. I could not tell whether the fluoride glass lens shown in their photo is aspheric or not (as is sometimes used in oil spaced refractors), but this material is difficult to precisely grind compared to crown glass, since it is a flouride of metals suspended in a glass matrix it has a quite high thermal expansion coefficient and is soft. If you try to naively polish it like normal glass, the surface often gets foggy, it is a bit weak against moisture and fragile. One cannot generalize, but the CNC machines for metal pieces often need a precision of +/-5-10 um only. Optical CNC machines that allow grinding and polishing of optics of similar size generally must have 10-100 times higher precision and e.g. the spindle rotation speed of the tool are higher than those used in metalwork, it will cost 5 to 20 times more as far as I know, with measurement instruments that characterize the optic in situ. It is a very different type of machine.

I think these newest devices really make the manufacturing of optics automatic (not necessarily higher quality), and little human intervention or expertise is needed compared to traditional methods; this is an important reason why the telescopes are so cheap and high quality, as in binoculars or cameras. This type of approach also allows companies to quickly move whole production facilities to new countries and hire the local people, many of whom have no prior experience with CNC or optics and cannot speak English (so training possibilities are limited). Some of the most attractive countries I heard now are in southeast Asia, as China is becoming expensive, even one of the big camera manufacturers moved a major base to Vietnam. I need to avoid drifting to geopolitics.

When I was in school, drafting on paper was one of the most important subjects, the instructor emphasized the need to visualize how the machining and measurement will be done and to orient the drawing accordingly. Also we learned machining using traditional lathes and mills, and there were skilled machinists who they said were 10 times more productive in high precision work compared to the average person. As far as I understand, this type of Decker devices are not available new anymore, all the precision manufacturers practically offer only CNC. Some young people just out of school do not know how to draft in 2D anymore. I do not want to generalize, but since the last 3-4 years, some of my outsourcing firms refuse to take anything other than a 3D CAD file; the traditional 2D drawing is practically ignored, and the companies who accept such legacy formats can ask for a premium because it increases their workload. They plug my file into a CAM program which semi-automatically sets up the machining sequence, even the tooling selection, and then this gets sent to their CNC machine via their LAN network. The CNC is optimized to take only one type of alloy or glass type. If I ask for a slightly different alloy of different hardness, they can refuse the job because it reduces their productivity, some human must open the CNC and change the tooling set or adjust the pitch etc. and this is too problematic. It is so competitive in certain parts of the commercial sector, I get intimidated. But the prices are extremely unbelievably low, because they must compete with foreign imports (note this generalization is not necessarily true for for all machining companies, but when you try to optimize cost-performance it often gets into such situations; the idea is to replace labor costs with automation to continue doing business at home). When I get the optics delivered in only a few weeks, some of them are measured with an interferometer to independently verify the specifications, and it is not very often the case nowadays that they are defective unless some exotic material is used. So this kind of highly automated approach actually works and seems to be increasing.

I admire the traditional approaches, I cannot interpret a Ronchi pattern like that.

After the discussion in this forum I bought two mass-produced 8 inch mirrors for fun, the first one from a supplier after asking for a particularly good copy, the second one anonymously via internet shopping. The first one arrived last week and I had time to study it during the weekend, I don't want to post any irresponsible numbers because I haven't spent too much time on this and I could make mistakes, also I don't think it's fair to the manufacturers, so don't trust what I say.

I do think the quality of the mass produced optic is far far far better than 35 years ago. That said, I do detect by interferometry a slight rotational assymmetry in the mass-produced mirror which would cause astigmatism. This assymmetry appeared to change in degree when the temperature of the optic was changed, though I cannot vouch for this because I did not bother to measure this many times, it takes hours. In the premium optician mirror I cannot detect such an rotational assymmetry within the sensitivity of my crude measurement. In the premium optic when the temperature is changed, there is a well-behaved spherical and rotationally symmetric deformation which arises and then diminishes as the temperature of the optic homogenizes, probably from the outside circumference inward (my interferometer measures relative height only so it is hard for me to tell, also it may be risky to interpret these temperature changing data; since the back side of the mirror is in thermal contact with a flat metal surface). Thermal time constant under my laminar flowbox happened to be like 2 hours, the premium optic has a shorter time constant because it is 5 millimeters thinner than the mass produced one. Though I do not want to put a number on it, the premium optic is closer to a symmetric parabolic shape.

The second thing I notice is larger apparent scattering in the mass-produced mirror compared to the premium one, when irradiating it with a laser beam and a photodetector positioned off-axis. In my experience this type of scattering normally means the surface is microscopically rough, but I cannot tell whether that is due to surface contamination, the roughness in the glass substrate, or the coating (the mass-produced mirror is enhanced aluminium so it will have 4 or more layers of interferometric coating). I did blow dry clean nitrogen on the surfaces prior to measurement, but I did not want to risk trying to clean too aggressively and possibly roughing it up even more, so I cannot vouch how much this is due to the mirror itself, and how much due to any surface contamination or scratches it might have accumulated on its way to me. I could gain more insight with a stylus measurement but I thought that was getting exaggerated.

The mirror is already coated, so I tried to look at stress-induced(?) birefringence in the base material by shining a linearly-polarized laser beam though the substrate and observing with an analysis polarizing prism pair, using the ratio of the two polarizations and normalizing appropriately. The interfrometric coating affects the polarization, I can only determine relative changes in birefringence (and I need to isolate the effect of the coating by making rough measurement using another laser beam from the front side of the mirror). Nevertheless variations in birefringence (it seems) were seen towards the edges of the optic in a characteristic, non-rotationally symmetric curve, more than I am used to for crown glass used in lenses or prisms. I'm not very sure how this arises. I irresponsibly speculated that one possibility is that the substrate was subjected to a relatively fast passage through the annealing temperature after being either form cast or water-jet cut into its disk form (please correct me if I am saying nonsense). If the glass has significant inhomogenity of composition in the base material, one can have spatial differences in the thermal expansion coefficient, but this effect is normally insignificant because the glass would be very well homogenized during the fabrication process. Since the supplier is known for producing refractor lenses of high quality, I guess they can be purposefully cutting corners for the material used in their Newtonian mirrors(?) to reduce customer cost, this might be a valid engineering decision perhaps. The premium optician mirror has much smaller variations in birefringence by comparison, similar to the grade used in some prisms and lenses, when I compare with similarly coated optics.

All these are small details, and I'm not sure whether they would have significant visible effects at the eyepiece, I'm not a very good critical observer. I will try to test it under the stars next week..... all this is for fun, and we are speaking of small differences, not brazen defects.

That said, I do detect by interferometry a slight rotational assymmetry in the mass-produced mirror which would cause astigmatism. This assymmetry appeared to change in degree when the temperature of the optic was changed, though I cannot vouch for this because I did not bother to measure this many times, it takes hours. In the premium optician mirror I cannot detect such an rotational

Residual stress-induced birefringence can be caused by many things as far as I have heard, one is when lots of glass are annealed and cooled in a hurry (the furnace usage cost will be charged by the hour, and it is very difficult to get a homogeneous temperature in the whole furnace when it is loaded with a lot of massive glass), some part of the substrate cooled too rapidly or did not heat up more than others, also depending on how it was supported if it were stacked in the furnace; sometimes if you support it with a rectangular spacer, the glass or ceramic at the bottom of the stack tends to take on a rectangular stress mark. I do know from personal experience that this can tend to become a problem in mass production; the prototype works but the latter mass quantity run does not. Perhaps better quality would either need to reduce the amount of glass loaded into the furnace or to take longer processing time, or to use a smaller well-controlled furnace, but I guess this is going in the direction of increased cost.....

Another possibility is that if the glass is cast rapidly, the consistency can become a bit non-uniform, this causes pockets with small variations in the thermal expansion coefficient, so that when it is cooled down, some parts of the glass contract more than others, leading to stress build-up. But I guess this latter possibility is very unlikely for borosilicate glass, unless a really low-grade glass or abbreviated production process were used.

In any case, internal stress does not necessarily lead to deformation, I guess gross defective copies can be rejected by quality control, this is a very valid way to produce low cost / high performance optics I guess. Some corner has to be cut to reduce costs after all, if lens quality glass were used I guess the price would increase by several times.......

No since I am so lazy, for many components I a): buy annealed substrates with inclusion and homogeniety specifications guaranteed by the glass manufacturer (if you anneal glass, the refractive index or dispersion can subtly change, going too slow through the anneal temperature compared to the standard stuff can shift the refractive index too far out compared to the standard material. I've never tested it, but I guess the optical glass annealed by premium mirror manufacturers can in principle have refractive indices that are out-of-spec compared to the standard material because of the prolonged annealing, this is fine for mirrors because who cares about refractive index, the lack of internal stress is the most important)..... and b): send it to the optical workshop. If you let the optical workshop use the materials that they have on stock, even if you trust them implicitly, there is more of a risk in terms of quality control later on when you want to produce the same component; because glass is often specified only by Abbe number and refractive index, often it has the same tradename (e.g., "BK-7 equivalent") but its composition, hardness, density can be quite different, especially after the eco glass changes...... crown glass is not so bad, but there are huge variations in flint glass.

What is impressive to me is that these telescope mirror companies can handle these lower-cost, internally-stressed materials and get a sorta-ok result. This is also a kind of price/technological innovation....... anybody stupid like me can throw money at the problem and use the best materials, but then it would cost too much......

...... Actually to really claim to "see" a difference between several 8 inch F5 mirrors at a level of p-v=lambda/4 etc., I may be wrong but the eyepiece axis and the primary mirror axis has to be aligned on the focal plane to within say 0.4 mm. A radial misalignment of 1 mm is equivalent to reducing the Strehl ratio of the primary to 0.8, the collimation error is introducing more coma and perhaps astigmatism when seen through an eyepiece, than any small defect on the mirror. Misaligning any of my alignment screws by 1/4 turn might be too much. My high-power eyepiece sits in the focuser with a play of 0.15 mm. A 1.2 meter long aluminium OTA will shrink by 0.3 mm when the temperature changes by 10 degrees C. Because of internal stresses in the tube it will also flex and deflect. The thin vanes in the spider and the mirror holder will flex too. If the laser beam used for collimation is internally misaligned or the center spot is not at the precise center of the paraboloid mirror, or the autocollimator has an internal defect, this can all prevent alignment to the necessary precision......

The OTA I built quickly for fun in 1 day without thinking isn't going to cut it, the clearance in the mounting holes for the mirror cell and spider are 0.3 mm larger than the retaining screws. It's a thin aluminium tube that is rolled and welded, and I can see that when I tilt it there is flexture because the focuser is too heavy, when I add up the buckle, flexture, and play I'm afraid I will bust this 0.4 mm error budget..... I need thicker seamless material or reinforcements.

Maybe I'm wrong, but an excellent mirror only can show its true colors if coupled with a rigid OTA that provides the necessary precision, and the user spends the time to really align it? In this sense, it is insufficient to talk only about the mirror, the question should actually be, "Would you trade a premium mirror in a premium OTA with a larger-aperture consumer-grade mirror in a consumer-grade OTA". Unless you were well versed in collimation by star testing, you might also need collimation tools, I'm not sure a multimode diode laser with an aspheric lens mounted on an eyepiece focuser alone is going to give the necessary precision to reliably get l/4 wave equivalent alignment if the two mirrors are severely misaligned........ though it's a really useful tool and I like it very much.

No since I am so lazy, for many components I a): buy annealed substrates with inclusion and homogeniety specifications guaranteed by the glass manufacturer (if you anneal glass, the refractive index or dispersion can subtly change, going too slow through the anneal temperature compared to the standard stuff can shift the refractive index too far out compared to the standard material. I've never tested it, but I guess the optical glass annealed by premium mirror manufacturers can in principle have refractive indices that are out-of-spec compared to the standard material because of the prolonged annealing, this is fine for mirrors because who cares about refractive index, the lack of internal stress is the most important)..... and b): send it to the optical workshop. If you let the optical workshop use the materials that they have on stock, even if you trust them implicitly, there is more of a risk in terms of quality control later on when you want to produce the same component; because glass is often specified only by Abbe number and refractive index, often it has the same tradename (e.g., "BK-7 equivalent") but its composition, hardness, density can be quite different, especially after the eco glass changes...... crown glass is not so bad, but there are huge variations in flint glass.

What is impressive to me is that these telescope mirror companies can handle these lower-cost, internally-stressed materials and get a sorta-ok result. This is also a kind of price/technological innovation....... anybody stupid like me can throw money at the problem and use the best materials, but then it would cost too much......

Having tested a number of imported optics, it’s not surprising how they keep the price low. Test the center, test the outer zone, if it doesn’t fit the -1 conic 1/4~ shell ship it anyway, it’s spent enough time on the machine.

ive heard Hubble optics have one person polishing, another comes by and tests it and then instruct the bench person what to do. My Hubble optic mirror is very nicely figured.

Having tested a number of imported optics, it’s not surprising how they keep the price low. Test the center, test the outer zone, if it doesn’t fit the -1 conic 1/4~ shell ship it anyway, it’s spent enough time on the machine.

ive heard Hubble optics have one person polishing, another comes by and tests it and then instruct the bench person what to do. My Hubble optic mirror is very nicely figured.

I can't criticize anybody right now because, a): I don't know how to figure a mirror except by writing a program and running on a CNC or farming it out, b): I can't figure out how to align the mirror to get out the l/4 wave performance.

It's a 202 mm diam paraboliod mirror, not a spherical one, so it's critical to know where the optical axis is because any tilt of the mirror >1 mm or axial mismatch >0.4 mm would introduce coma that would not be negligible compared to the wavefront distortion of l/4 if I'm not mistaken. But I don't know how to determine the optical axis, I have to assume that it coincides with the geometrical center of the mirror. There is a mark there, but this is a flexible sticker, I don't think it can have say +/- 0.1 mm precision (?). Also I don't think the mirrors have 0.1 mm true round dimensions, the OD looks rough-cut. Now I use a sight tube+autocollimation tool to align by eye (this assumes I can align within 0.4 mm a mark which is placed 1 m from my eye, but my right eye has some astigmatism and I am nearsighted). I verified my alignment by mounting the OTA on an optical table at an angle 20 degrees from horizontal, and aligned using my eye and commercial collimation tools in a bright room. Then I used a single-mode helium neon laser (this type of laser has what is called "beam pointing (in)stability" issues too, when the beam drifts or walks off in the first hour after turning it on because the resonator warms up, but let's say it's better than a pocket diode laser, plus it retains a strict Gaussian mode profile even after propagating 10 meters), aligned through the axis of the focuser within a precision of say +/-0.1 mm using a round adjustable iris+removable pinhole (to prevent embarrassing diffraction misalignments?), through the center of the mark on the mirror measured with a caliper to within +/-0.2 mm, back again through the focuser, and projected on the wall 3 meters from the scope........ And it was barely within the tolerance needed. Maybe if I gain enough experience...... but if it's done in the dark, I'm not sure I can align properly. Well I'm pretty sure I can't, I have to figure it out.

I picked up the OTA, moved it to the end of the room, put it back on the optical table, and that was enough to mess up the collimation waaaaay out of the l/4 requirement. I stuck a piezo element on the side of the OTA and vibrated it, and I could see that the OTA doesn't have enough rigidity to keep the l/4 dynamically either, probably tapping on the side of the OTA is enough to momentarily make it lose collimation. I left the telescope and came back after 6 hours, and the laser spot projected on the wall had subtly moved.

I have to stop playing with telescopes, but I think the aluminium tube walls have to be pretty thick to prevent these deflections, I wanted a lightweight telescope but I guess I'm not getting one (carbon..... fiber?). Sonotube (here there's hard-paper tubes but the wall thickness is pretty thin) I guess has thermal expansion coefficient half of aluminium, the rigidity seems to be quite high, the old methods were pretty good to begin with. I think I ask some supplier to machine some tube(s) of various materials for me, I'm too lazy to do it myself, then after a few weeks I will try again........ the area around the focuser and the mirror cell have to be reinforced maybe....

In this sense, a 6 inch F8 mirror has way (millimeter scale) easier tolerances in terms of alignment and mechanical rigidity of the OTA, plus it's much lighter......... you would not need to worry half so much, to get out the maximum performance every day..... I dunno, short focal length large diameter mirrors in a lightweight tube that is cheap, easy to align and has <l/10 diffraction limited performance...... there are conflicting requirements. I guess it's not a realistic expectation.

Okay, the CNC probably won't adjust when the pitch goes fickle and 30 seconds later results in a setback of hours - not that it's ever happened to me. The variables of figuring to millionths of an inch sometimes require a multifaceted effort - could be the glass expansion/anneal or humidity and temperature not to mention the substrate or tool support. Opticians don't even agree on faceting a pitch lap. I know Tex's approach to making a lap is miles away from Dobson's. Spherical optics can be setup for machine production but size does matter in optics, IMHO. A quarter wave is very small; a tenth wave is much smaller and frustrating. I'm talking about smooth aspherical correction - rms is a better analysis but requires sophisticated testing.

This is just my opinion (amongst many) but I base it on personal experience and struggles involving the differences with 1/2 wave and somewhat better astronomical optics. So take it for what it's worth I guess. Consumer scopes and optics are much better than decades ago. But I have good and excellent scopes and I know which I prefer in good seeing.

I can't criticize anybody right now because, a): I don't know how to figure a mirror except by writing a program and running on a CNC or farming it out, b): I can't figure out how to align the mirror to get out the l/4 wave performance.

It's a 202 mm diam paraboliod mirror, not a spherical one, so it's critical to know where the optical axis is because any tilt of the mirror >1 mm or axial mismatch >0.4 mm would introduce coma that would not be negligible compared to the wavefront distortion of l/4 if I'm not mistaken. But I don't know how to determine the optical axis, I have to assume that it coincides with the geometrical center of the mirror. There is a mark there, but this is a flexible sticker, I don't think it can have say +/- 0.1 mm precision (?). Also I don't think the mirrors have 0.1 mm true round dimensions, the OD looks rough-cut. Now I use a sight tube+autocollimation tool to align by eye (this assumes I can align within 0.4 mm a mark which is placed 1 m from my eye, but my right eye has some astigmatism and I am nearsighted). I verified my alignment by mounting the OTA on an optical table at an angle 20 degrees from horizontal, and aligned using my eye and commercial collimation tools in a bright room. Then I used a single-mode helium neon laser (this type of laser has what is called "beam pointing (in)stability" issues too, when the beam drifts or walks off in the first hour after turning it on because the resonator warms up, but let's say it's better than a pocket diode laser, plus it retains a strict Gaussian mode profile even after propagating 10 meters), aligned through the axis of the focuser within a precision of say +/-0.1 mm using a round adjustable iris+removable pinhole (to prevent embarrassing diffraction misalignments?), through the center of the mark on the mirror measured with a caliper to within +/-0.2 mm, back again through the focuser, and projected on the wall 3 meters from the scope........ And it was barely within the tolerance needed. Maybe if I gain enough experience...... but if it's done in the dark, I'm not sure I can align properly. Well I'm pretty sure I can't, I have to figure it out.

I picked up the OTA, moved it to the end of the room, put it back on the optical table, and that was enough to mess up the collimation waaaaay out of the l/4 requirement. I stuck a piezo element on the side of the OTA and vibrated it, and I could see that the OTA doesn't have enough rigidity to keep the l/4 dynamically either, probably tapping on the side of the OTA is enough to momentarily make it lose collimation. I left the telescope and came back after 6 hours, and the laser spot projected on the wall had subtly moved.

I have to stop playing with telescopes, but I think the aluminium tube walls have to be pretty thick to prevent these deflections, I wanted a lightweight telescope but I guess I'm not getting one (carbon..... fiber?). Sonotube (here there's hard-paper tubes but the wall thickness is pretty thin) I guess has thermal expansion coefficient half of aluminium, the rigidity seems to be quite high, the old methods were pretty good to begin with. I think I ask some supplier to machine some tube(s) of various materials for me, I'm too lazy to do it myself, then after a few weeks I will try again........ the area around the focuser and the mirror cell have to be reinforced maybe....

In this sense, a 6 inch F8 mirror has way (millimeter scale) easier tolerances in terms of alignment and mechanical rigidity of the OTA, plus it's much lighter......... you would not need to worry half so much, to get out the maximum performance every day..... I dunno, short focal length large diameter mirrors in a lightweight tube that is cheap, easy to align and has <l/10 diffraction limited performance...... there are conflicting requirements. I guess it's not a realistic expectation.

You're right, of course, about how critical (and how possible) the exact alignment is with fast paraboloids, but coma correctors will take care of that problem.

I just played a little with tilting the 12.5" f/5 mirror in OSLO, with my coma corrector, designed from off the shelf lenses.

I get polychromatic Strehl 0.8 with original focus misalignment of 9.5mm. This is angular tilt of the mirror of 0.34 deg.

What happens is:

1) Just like without a corrector, the best image moves off axis at the focal plane, to roughly about: linear misalignment x corrector magnification.

2) If the corrector, when everything is perfectly aligned, makes a diff limited image of a certain radius, roughly, that radius divided by corrector magnification is the linear measure of misalignment you can make and still have a diff limited image at the center of FOV.

All in all, my little coma corrector, will chew up the usual misalignment errors without anybody noticing it.

I bet that Paracorr can do the same only with much faster mirrors and more alignment error.

You're right, of course, about how critical (and how possible) the exact alignment is with fast paraboloids, but coma correctors will take care of that problem.

I just played a little with tilting the 12.5" f/5 mirror in OSLO, with my coma corrector, designed from off the shelf lenses.

I get polychromatic Strehl 0.8 with original focus misalignment of 9.5mm. This is angular tilt of the mirror of 0.34 deg.

What happens is:

1) Just like without a corrector, the best image moves off axis at the focal plane, to roughly about: linear misalignment x corrector magnification.

2) If the corrector, when everything is perfectly aligned, makes a diff limited image of a certain radius, roughly, that radius divided by corrector magnification is the linear measure of misalignment you can make and still have a diff limited image at the center of FOV.

All in all, my little coma corrector, will chew up the usual misalignment errors without anybody noticing it.

I bet that Paracorr can do the same only with much faster mirrors and more alignment error.

My brain is not working now, but is that right?

Suppose the axis of the mirror and the axis of the eyepiece are now radially misaligned, translated by 1 mm because I messed up with my collimation procedure. We next get happy and install the coma corrector in between; but unbeknown to me this corrector will also be misaligned with respect to the true optical axis of the paraboloid mirror by 1 mm (because the comma corrector unit is physically threaded, fixed to the "misaligned" focuser). Will now this misaligned coma corrector....... improve the induced aberration for the 1 mm area of the image around the radial misalignment?

Doesn't the misaligned coma corrector instead make the aberration worse for this 1 mm area, "correcting" the coma in the wrong direction, then this part of the degraded image circle gets magnified by the short focal length eyepiece when you look at Jupiter?

I get the impression, the coma corrector is relatively critical in terms of axial alignment, not so forgiving..... and maybe also in terms of placing it at the correct design distance from the mirror. Otherwise the amount of coma correction (or even its direction) might shift away from the design values. But maybe it's my imagination.

I think p-v=1 lambda surface error is for sure possible with a sub-diameter tool on a high-end CNC (a low cost laser interferometer mounted on a vibrating optical table using phase shifting interferometry often cannot measure mid spatial frequency errors so well anyway).

p-v 1/2 lambda surface error (NOT wavefront error, that would be p-v 1 lambda), maybe also with a sub-diameter tool. Maybe. I would not dare write that down on the specs.

p-v 1/4 lambda, I admit I don't know how to do that with a CNC, particularly not with borosilicate glass.

But because I don't know how to collimate such a precise mirror in my telescope yet to that level of precision where it would matter, I'm not at that stage yet in multiple fronts. Even if they made such a precise mirror each time, if they use an imprecise OTA that cannot hold that level of collimation, or they used low-cost alignment tools, it would not seem to make sense to make the customer pay for that.......?